The present invention relates to a novel method of preparing alkoxysilanes and oligomeric alkoxysiloxanes. The method comprises reacting a metal silicate with an acid selected from the group consisting of sulfurous acid and acids with a pKa greater than about 2.5 in the presence of an alcohol. The resultant product is then reacted with an alcohol to form the alkoxysilane or oligomeric alkoxysiloxane, depending on the starting silicate. This method is particularly valuable since the reaction conditions are mild and the reactants are readily available.
Several methods of producing alkoxysilanes are known in the art. The most well known and often used of these methods involves the reaction of silicon tetrachloride with an alcohol. Despite the high yields which can be obtained, this method is disadvantageous in that it is a 2-step process initially requiring the formation of silicon tetrachloride.
In order to avoid this roundabout approach, a method has been developed that yields alkoxysilanes by a 1-step reaction using elemental silicon and alcohols. Unfortunately. elemental silicon is generally produced from silica by a very endothermic and thus costly reaction.
Several routes to oligomeric alkoxysiloxanes have also been developed. For instance, in one route silicon tetrachloride is treated with a limited amount of water to form a reaction mixture from which an appropriate chlorosiloxane is isolated. The chlorosiloxane is then treated with an alcohol to yield the alkoxysiloxane. Such routes, however, often yield mixtures from which it is difficult to separate the desired alkoxysiloxane.
Kenney et al. in U.S. Pat. No. 4.717.773 teach an alternative route to alkoxysilanes and oligomeric alkoxysiloxanes comprising reacting a metal silicate with a strong acid-alcohol solution and then esterifying the resultant product with an alcohol. The only acids taught in this work, however, are strong acids such as HC1.
The present inventors have now discovered that alkoxysilanes and oligomeric alkoxysiloxanes can be made by reacting metal silicates with an acid selected from the group consisting of sulfurous acid or acids with a pKa greater than about 2.5 and then esterifying the resultant product with an alcohol.